Re: Tesla semi?
Regenerative braking is essential if the economics of electric power are to be realised in everyday stop-go driving on real roads.
Basically, when braking the motor acts in reverse as a dynamo and charges a supercapacitor. When accelerating, you drain the supercapacitor before drawing on the battery. In F1 racing the system is called CURS.
Also, you won't even need to use the brake pads unless braking sharply, as the dynamo exerts its own very effective braking force, so you greatly reduce copper and/or asbestos pollution of the roadside.
How good the Tesla toys are at that depends on how good their designers are, but if F1 can do it routinely I don't see why Tesla can't.